The present invention relates to a catalyst for efficiently decomposing methanol to produce hydrogen and an apparatus comprising such a methanol-decomposing catalyst.
Though most of energy sources for automobiles have conventionally been petroleum fuels such as gasoline, diesel oil, etc., alternative energy sources have been attracting much attention in view of the depletion of petroleum oil and various environmental problems such as warming of global climate, acid rain, etc. Such alternative energy sources are non-petroleum engine fuels such as a natural gas and methanol, and batteries and fuel cells for electric vehicles, and some of them have already been practically used.
Engines running with non-petroleum fuels such as a natural gas and methanol are advantageous in less emission of CO2 and noxious gases such as NOx than the gasoline or diesel engines. However, the non-petroleum fuels are still derived from limited sources of fossil fuels.
Electric vehicles free from the problems of burning fuels generally run with excellent energy efficiency without emission of exhaust gas polluting the environment. However, the electric vehicles running by batteries are disadvantageous in many respects such as limited distance covered by one charge and a long charging time of batteries, which should be solved for cars running streets.
In view of such circumstances, the electric vehicles running with fuel cells utilizing an electrochemical reaction between hydrogen and oxygen have attracted much attention as alternatives to the prevalent engine vehicles. The electric vehicles with fuel cells are advantageous in energy efficiency free from the problems of exhaust gas. However, a hydrogen gas should be carried and supplied safely and efficiently in the fuel cells. It is thus recently proposed that methane or methanol is used as a hydrogen gas source to be decomposed to generate a hydrogen gas while running. In particular, methanol is most promising as a hydrogen gas source, because methanol is in a state of liquid that can easily be stored in a tank and supplied like gasoline.
When methanol is decomposed to produce a hydrogen gas, the following reactions:
CH3OHxe2x86x92HCHO+H2, and
HCHO+H2Oxe2x86x92CO2+2H2
mainly occur with inevitable side reactions generating by-products such as carbon monoxide. Because a gas produced by methanol decomposition is directly introduced into a fuel cell, the methanol decomposition gas should have as high a hydrogen concentration as possible with minimum concentrations of by-products and unreacted methanol.
The amount of a hydrogen gas required changes depending on whether or not and how automobiles are accelerated, leading to change in the amount of a fuel to be decomposed such as methanol and thus the temperature of the catalyst in a range of 200 to 600xc2x0 C. Accordingly, the methanol-decomposing catalyst should have activity that little lowers for a long period of use in such a temperature range, and high thermal stability. Further, the methanol-decomposing catalyst should be maintenance-free because it is used on vehicles.
Conventionally known as such methanol-decomposing catalysts are (a) catalysts produced by compressing catalytically active components such as oxides of copper, zinc, etc. into pellets with binders such as graphite; and (b) catalysts comprising noble metals such as palladium as catalytically active components. However, the catalysts (a) palletized with graphite do not have sufficiently large contact area with methanol, failing to achieve efficient decomposition of methanol in a small catalyst amount. The catalysts (b) comprising noble metals such as palladium as catalytically active components are poor in selectivity for generating hydrogen.
Both catalysts suffer from the problem that the resultant gas does not contain a sufficiently high concentration of a hydrogen gas while containing relatively large amounts of by-products such as CO, etc. Thus, it is difficult to make smaller a methanol-decomposing apparatus for generating a hydrogen gas that is to be supplied to a fuel cell.
Accordingly, an object of the present invention is to provide a methanol-decomposing catalyst having excellent durability at widely changeable temperatures for efficiently producing a reaction gas having a high hydrogen gas concentration with sufficiently suppressed by-products, and a methanol-decomposing apparatus comprising such a catalyst.
As a result of intense research in view of the above object, the inventors have found that a methanol-decomposing catalyst comprising catalytically active components comprising copper and zinc, and a carrier for supporting the catalytically active components and composed of zirconia and/or titania together with ceria can efficiently decompose methanol to produce a hydrogen gas while sufficiently suppressing undesired side-reactions. The present invention has been completed based upon this finding.
The methanol-decomposing catalyst according to the present invention comprises catalytically active components of copper and zinc supported by a carrier composed of zirconia and/or titania together with ceria. Ceria added to the carrier acts to suppress the side reactions, contributing to efficient production of a hydrogen gas.
The ceria content is preferably 5 weight % or more based on the -carrier to sufficiently suppress the side reactions. Copper and zinc are preferably present in the form of oxide on the carrier. To achieve high efficiency in the production of a hydrogen gas, a weight ratio of copper/zinc is preferably {fraction (1/10)} to {fraction (10/1)} on an oxide basis.
The methanol-decomposing apparatus according to the present invention comprises the above methanol-decomposing catalyst and a means made of ceramics or metals for supporting the catalyst.